The present invention relates to optical apparatuses that use the near-field light, such as optical microscopes, optical measurement instruments, spectroscopic instruments, and optical recording/reproduction apparatuses.
In the conventional optical microscope, light is focused with lenses. In this case, the spatial resolution is limited by the light wavelength.
On the other hand, the near-field optical microscope uses a probe having a microstructure whose dimensions are of an order of nanometer, for example, a micro aperture whose diameter is of an order of nanometer. When the microstructure is irradiated with light, localized light called near-field light is generated in the vicinity thereof. If the microstructure is brought close to a sample and the object (the sample) is illuminated with the near-field light, the localized near-field light is converted to propagating light, depending on the complex refractive index of the sample, that can be observed by a light detector at a distance. Since its intensity depends upon optical characteristics of the sample, scanning the microstructure on the surface of the sample makes it possible to measure the optical characteristics of the sample with a spatial resolution determined by the size of the microstructure. Recently, this technology has begun to be applied to a wide range of fields, such as various optical measurements, high-density optical recording, and light machining.
A near-field light probe that is most widely used is a sharply pointed optical fiber with a metal cladding and having an aperture smaller than the light wavelength on a top end thereof. However, generation efficiency of the probe is low. For example, in the case of an optical fiber having a micro aperture of 100 nm, the light intensity emitted from the fiber is 0.001% or less of the light intensity that has entered the fiber (1-1 Applied Physics Letters, Vol. 68, pp. 2612-2614, 1996.) This low level of efficiency becomes a problem when the near-field light technologies are applied to various fields.
Thus, to increase the generation efficiency of the near-field light, methods that use localized plasmon excitation in metals have been proposed, as in the following. That is, there are: (1) a method that uses a metal probe of the scanning tunnel microscope (1-2, “Unexamined Patent Application Laid-Open, No. H06-137847”) (2) a method that uses a metallic micro sphere provided on the center of the aperture of the micro aperture fiber probe (1-3 “Unexamined Patent Application Laid-Open, No. H11-102009”); (3) a method that uses a probe where metal scattering members are provided on a button surface of a glass substrate (1-4) “Unexamined Patent Application Laid-pen, No. H11-250460”); (4) a method that uses a triangular metal pattern provided on a flat substrate (“Unexamined Patent Application Laid-Open, No. 2000-73922” applied for by the same applicant as that of this application); (5) a method that uses a quadrangular pyramid with metal films formed on two side faces thereof, (1-5 “Technical Digest of 6th International Conference on Near Field Optics and Related Techniques”, the Netherlands, Aug. 27-31, 2000, pp. 100”). Compared to the above-mentioned conventional methods (1) through (3), the conventional (4) can yield intense near-field light and its fabrication is easy.
When a scattering-type probe is used, removal of background light is important, as will be described later. As means for achieving this, (b) a method whereby the probe is vibrated and only the optical signal that is synchronized with the vibration is detected is disclosed in “Unexamined Patent Application Laid-Open, No. H06-137847.” Further, as another means, a method whereby a difference between amplitudes of two mutually orthogonal polarized lights is taken is disclosed in “Unexamined Patent Application Laid-Open, No. 2000-298132.”
The above-mentioned conventional methods (1) through (5) each realize generation of the intense near-field light, but contain a problem in that light passes through a part other than the metallic surface and acts as background light, which brings on a decrease in the S/N (signal-to-noise) ratio of the detected signal.
Use of localized plasmon excitation can increase the square of an absolute value of the electric field strength of the near-field light, namely the photon density per unit area. However, the total number of photons of the near-field light, namely a value obtained by integrating the above-mentioned photon density for a localized area of the near-field light, becomes not so large because of the small area resulting from the objective of increasing the spatial resolution.
Conversely, although the background light is small in comparison to the electric field strength, it has an area larger than the diffraction limit; therefore, it is often the case that the total number of photons becomes measurably large. Assuming that the area of the near-field light is 1/S times the area of the background light (5) and the absolute value squared of the electric field strength of the near-field light is G times that of the background light, the total number of photons of the near-field light becomes G/S times that of the background light For example, in the above-mentioned method (4), the absolute value squared of the electric-field strength of the near-field light that is localized in an area of 5 nm by 5 nm is 5700 times that of the incident light, but the area of the background light, whose number of photons is equivalent to the number of photons of this near-field light, becomes 380 nm by 380 nm.
This value is not more than the diffraction limit of the near-infrared laser light commonly used as a light source, and consequently, with a normal device configuration, the number of photons of the background light becomes larger than that of the near-field light. In the case where nonlinear interaction is used as in optical recording, this background light causes no problem because the interaction is affected not by the total number of photons, but by the number of photons per unit area. However, in the case, such as normal optical reproduction and the near-field optical microscope, where a portion of the near-field light is scattered by the sample and the amount of the light entering a detector is observed, the background light similarly enters a detector, and hence, the S/N ratio (signal-to-noise ratio) becomes smaller than unity. Also, in the case where secondary light from the sample, such as fluorescent light and Raman light, is observed as signal light, the same problem is involved because secondary light generated by the above-mentioned background light becomes a background signal.
Up to this point, a simple comparison of the number of photons in the vicinity of the sample was considered. Besides, in the above-mentioned conventional methods (1), (3), and (5), the background light is propagating light, the background light enters the detector generally with a higher degree of efficiency compared to the efficiency at which the near-field light, that is non-propagating light, is scattered and enters the detector placed in the distance, and hence the S/N ratio decreases further.
To solve this problem, in the conventional methods (2) and (4), the background light is suppressed by shading the periphery of a metal pattern for exciting localized plasmons with an aperture having dimensions not more than the light wavelength remained. However, when dielectric materials are used as shading materials, it is difficult to achieve a sufficient shading property, and further problems, such as generation of heat, may occur.
When a metal with a high shading effect is used to avoid these problems, the following problem occurs. If there exists a metal that has a plane parallel to a vibrating direction of the localized plasmons, an inverse electric field is generated inside the above-mentioned metal which inhibits the plasmon excitation. In other words, when a metal for shading that forms an aperture is brought close to the metal for exciting the localized plasmons, the excitation of the localized plasmons is inhibited, and, hence, the intensity of the near-field light is decreased. Further, depending upon the shape of the metal for shading and the direction of polarization of the incident light, the localized plasmons are exited also in the metal for shading to effect reduction in spatial resolution. Therefore, the aperture cannot be made sufficiently small, and, consequently, there is a limit to the amount that the S/N ratio can be compared. Moreover, since the dimensions of the excitation area of the localized plasmons are determined by the radius of curvature of a sharply pointed part of the metal, the metal top end needs to be sharpened in order to improve the spatial resolution, and, for this reason, the thicknesses of the metal for shading and of the metal for exciting the localized plasmons at the top end cannot be increased. Therefore, light passes through this part and becomes background light, which presents a problem in that it results in a decease in the S/N ratio all the same.
Moreover, in the above-mentioned conventional method (7), a technique is employed whereby the background light is removed through the interference of two mutually orthogonal polarized lights. However, since this method uses a scattering-type probe, a shading plate for shading the background light etc. cannot be installed due to its configuration, resulting in an increase In the absolute amount of the background light, and the wavefront of the background is deformed in consequence of the scattering by the sample and the probe as a whole; therefore, it is difficult to completely remove the background light merely using interference. In addition, the surface conditions of the sample govern how the wavefront is deformed, and, consequently, the elimination of the background light is also affected by the surface conditions of the sample.